The proposal is directed at studying neuroanatomical correlates of genetic sex throughout ontogeny in the canary song system. This system offers exceptional opportunities for relating steroid changes to neuroanatomy, and to specific behaviors. Singing in canaries is a sexually dimorphic behavior. Male canaries sing during the breeding season. Females normally do not sing but will do so if treated with androgens as adults. Canary song is learned: a bird will not sing normal song as an adult unless exposed to song while a juvenile. Singing is controlled by several discrete and clearly demarcated brain nuclei. These nuclei concentrate androgens and are substantially larger in male canaries than females. Thus, the effects of hormonal status, learning, and practice converge in relatively small brain regions to create the neuroanatomical substrate for song, a complex, species-typical behavior. In no other vertebrate system can sexual differences in neuroanatomy be so directly related to prior hormonal status and consequent behavioral effects. However, the nature and development of neuroanatomical sex differences in canaries have not yet been systematically studied. Two classes of experiments are proposed to study this neuroanatomical divergence between the sexes. First, the development of neuroanatomical dimorphisms in normal subjects will be measured. These differences in young birds should be part of the process whereby the brains of the two sexes become organized differently. Second, adult males in the breeding and non-breeding seasons will be compared. Androgen levels drop after the breeding season and the song control nuclei appear to shrink. This hormonally induced regression may be necessary for the subsequent reacquisition of song, perhaps by reinstating features of juvenile brain plasticity. The analyses of song control nuclei in each of these experiments will be carried out on tissue prepared for Nissl, Golgi and electron microscopic observation. Quantitative analyses in each case will make use of a computerized neuronal data acquisition system developed by the P.I.